CDs (Compact Discs), DVDs (Digital Versatile Discs), and BDs (Blu-ray (registered trademark) Discs) are commonly and generally known as conventional optical disks. In recent years, there has been a demand to further increase recording density. To allow implementation of an optical disk information device that enables high-density recording and reproduction, a reduction in the track pitch of optical disks or an increase in linear density is needed.
As described above, the track pitch of optical disks is effectively reduced in order to increase the recording density of the optical disk information device.
However, the reduced track pitch disadvantageously increases the amount of crosstalk in which signals recorded in tracks adjacent to a scanned track are added to a reproduction signal, leading to noise.
Thus, to solve this problem, some conventional optical disk information devices include a one-beam optical system that cancels crosstalk by performing an arithmetic correction on outputs from three areas into which a light receiving element is divided in a direction orthogonal to tracks as described in Patent Literature 1; the arithmetic correction is expressed by S=K•C+R+L (K is a constant) where a central signal is denoted by C, a right signal is denoted by R, and a left signal is denoted by L.
However, in the conventional configuration, that is, the optical disk information device in Patent Literature 1, when the track pitch is increasingly reduced in order to improve the recording density, tracking error signals may fail to be obtained due to a diffraction limit. That is, the reduced track pitch also reduces the groove pitch of guide grooves forming the tracks. When the groove pitch decreases below the diffraction limit (=λ(2•NA), where NA denotes numerical aperture), the amount of light returning to an objective lens stops changing, precluding tracking error signals from being retrieved from the guide grooves. Furthermore, even when the groove pitch is larger than the diffraction limit, a small difference between the groove pitch and the diffraction limit leads to a very small change in the amount of return light. This also prevents sufficient tracking error signals from being obtained.
For example, conditions for simulations and experiments described in FIG. 9 in Patent Literature 1 include a wavelength of 780 nm, an objective lens NA of 0.5, and a track pitch Tp of 0.8 μm. Under these conditions, the diffraction limit for light beams is 0.78 μm, and the track pitch is approximately equal to the diffraction limit. Thus, crosstalk is reduced but tracking error signals are not obtained, precluding information signals from being reproduced.
To allow sufficient tracking error signals to be obtained without affecting reproduction of information signals, the track pitch Tp needs to satisfy the following formula.Tp>1.2•(diffraction limit)=1.2•(λ/(2•NA))
However, for example, BDs, currently commercially available high-density optical disks, have a wavelength of about 0.405 μm, an objective lens NA of 0.85, and a track pitch of 0.32 μm. When it is assumed that the density is farther increased by reducing, the track pitch, then Tp>0.29 μm. The resultant scale factor of the increase in density is about 1.2, and thus, the effect is insignificant. Furthermore, due to the degree of the reduction in track pitch Tp, the amount of increase in crosstalk is small, and the effect of the introduction of the crosstalk canceller is very insignificant.
Therefore, in areas in which a substantially significant crosstalk cancel effect is exerted, sufficient tracking error signals are not obtained. The conventional configuration is thus inconsistent with the increase in density, which is the original object.